If leaving the RTC wakeup alarm on the BIOS worked with the old interface, you may need to disable it when migrating to the new interface.

If leaving the RTC wakeup alarm on the BIOS worked with the old interface, you may need to disable it when migrating to the new interface.

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{{Warning box|The wakealarm interface is incompatible with the kernel's old "Enhanced Real Time Clock Support" and "Generic /dev/rtc emulation" options. If your kernel was built with these enabled your kernel log will contain messages such as <pre>rtc_cmos: probe of 00:03 failed with error -16</pre> The solution is to rebuild your kernel with the above two options excluded (find them under Drivers -> Character Devices) and the various RTC interfaces (found under Drivers -> Real Time Clock) included. From a .config point of view CONFIG_RTC and CONFIG_GEN_RTC must be unset and, at a minimum, RTC_INTF_SYSFS must be set.}}

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{{Warning box|The wakealarm interface is incompatible with the kernel's old "Enhanced Real Time Clock Support" and "Generic /dev/rtc emulation" options. If your kernel was built with these enabled your kernel log will contain messages such as <pre>rtc_cmos: probe of 00:03 failed with error -16</pre> The solution is to rebuild your kernel with the above two options excluded (find them under Drivers -> Character Devices) and the various RTC interfaces (found under Drivers -> Real Time Clock) included. From a .config point of view CONFIG_RTC and CONFIG_GEN_RTC must be unset and, at a minimum, RTC_INTF_SYSFS must be set.

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If you're running a modular kernel, such as the stock Debian kernel, you can unload the rtc module and then load the rtc-cmos module.}}

BIOS Setup

You will need to have a motherboard that supports both ACPI and a Real Time Clock Alarm Functions in the BIOS. Dont forget to enable this in the BIOS. You will find a tab similar to "Advanced Power management".

The wake-up function is provided on most modern boards by the ability to "wake from RTC alarm" - setting a date and time in BIOS will cause the computer to power itself on and boot at the set time. This setting is likely found under a sub-menu of "Power Management" "APM", or "Advanced" - you just need to poke around to find it. It will likely be called something like Wake from RTC, Wake From Alarm, RTC Resume, etc.

Tip: On many boards, when this setting is enabled, it will wake only from a time set and saved from BIOS setup, and not from a time set outside of the BIOS setup environment - as we want. All of the boards the original author of this document needed this setting disabled to correctly wake with ACPI. This is the recommended starting point.

Testing ACPI Wakeup Works

Here are the commands to check that you have ACPI working.

$powersave -S
ACPI

Check that you have the alarm function in /proc/acpi/alarm. This prints out the RTC Alarm clock from the BIOS.

$cat /proc/acpi/alarm
2005-**-29 10:10:04

Check that you can write a new time to the RTC Clock Alarm (you will need to be Root)
Format is YYYY-MM-DD HH:MM:SS

#echo "2005-12-29 10:10:04" >/proc/acpi/alarm

Check that it was written, shut down and wait for it to wakeup.

#cat /proc/acpi/alarm
2005-*12-29 10:10:04
#shutdown -h now

Note: You may not see the change in the bios, but it should still work

Now go to Mythtv-Setup and setup wakeup and shutdown parameters.

Kernel 2.6.22+ and /sys/class/rtc/rtc0/wakealarm

Beginning with kernel 2.6.22 /proc/acpi/alarm has been removed and replaced with /sys/class/rtc/rtc0/wakealarm. The key differences with the wakealarm interface are:

Instead of accepting a formatted time, wakealarm accepts the number of seconds since Jan 1, 1970.

If you want to change the wakealarm time, you first need to reset the time.

You have to make sure that your bios clock is set to UTC time not localtime otherwise it will wakeup at the wrong time

This acpi-alarm script will function with the new sys wakealarm interface or the old acpi/alarm interface

If leaving the RTC wakeup alarm on the BIOS worked with the old interface, you may need to disable it when migrating to the new interface.

Warning: The wakealarm interface is incompatible with the kernel's old "Enhanced Real Time Clock Support" and "Generic /dev/rtc emulation" options. If your kernel was built with these enabled your kernel log will contain messages such as

rtc_cmos: probe of 00:03 failed with error -16

The solution is to rebuild your kernel with the above two options excluded (find them under Drivers -> Character Devices) and the various RTC interfaces (found under Drivers -> Real Time Clock) included. From a .config point of view CONFIG_RTC and CONFIG_GEN_RTC must be unset and, at a minimum, RTC_INTF_SYSFS must be set.

If you're running a modular kernel, such as the stock Debian kernel, you can unload the rtc module and then load the rtc-cmos module.

Trouble Shooting

Getting autostart using alarm can be quite frustrating. Here's some tips on getting it working.

UTC Bios Clock

Remember, your bios time might be in UTC rather than local time using:

# echo "+00-00-00 00:05:00" > /proc/acpi/alarm

will set the wakeup time to 5 minute from now, regardless of whether the RTC is in UTC or locatltime.

Since the wake-up time given by mythtv to the mythtv wakeup command is in local time, you need to do some bash handling of the obtained time to be able to setup the wakeup time in the bios in UTC (independent of daylight savings time and such). The following bash code might help here:

As you noticed the echo to the acpi-alarm is commented out by default, because it is really better to check first if everything is working correctly (time format the same on your bios and such).

An alternative script to the above is the following:

#!/bin/bash
stamp_file=/home/mythtv/timestamp
#just log what we get as command line parameters
echo $1 $2 $3> $stamp_file
#i setted mythtv to output the number of seconds since epoch
#so i calculate the number of hours, minutes and seconds from
#now the computer has to wakeup:
sfn=$(($2 - `date +"%s"`))
#and then send it to /proc/acpi/wakeup in the format we saw above
y=`(echo $(($sfn - 3600))|awk '{print strftime("+00-00-00 %H:%M:%S", $1)}')`
echo "$y">/proc/acpi/alarm
echo "$y">>$stamp_file
echo "executed at `date`" >> $stamp_file
exit

This works on a lot of different EPIA and ASUS Motherboards, and is not dependent on the timezone you use in the bios.

Fussy Bios

Some motherboard bioses are extremely fussy when setting the wake up alarm. These bioses refuse to wake up if the hardware clock is modified after the alarm timer has been set. Most linux distributions do this by default when shutting down (they write the current system time back to the hardware clock). This causes the system never to wake up.

Hence check your shutdown scripts!

Under ubuntu/debian modifying /etc/init.d/hwclock.sh with the following will fix this problem:

Another possible glitch is, that the option 'Resume By Alarm' (or whatever it is called) is set to Enabled but wake up using /proc/acpi/alarm only works if the option is set to Disabled. Sounds weird but works with some boards.

User experience

spent hours on ACPI on his MSI RS482M-IL board. Not working, system never wakes-up. (march 2007)

Mattwire 04 September 2007. Works fine with MSI K8NGM2-FID and NC6120 laptop running Ubuntu Feisty and Gutsy. Both require modification to /etc/init.d/hwclock.sh to save the wakeup time.

Dumdideldum 06 September 2007 Works with Epox 8rda+ running Ubuntu Feisty. It is important to disable the wakeup on rtc in the Bios and editing the /etc/init.d/hwclock.sh as described in the Ubuntu guide linked in the section below.

Pkendall 09 September 2007 Works fine with MSI K9NBPM2-FID running Ubuntu Feisty. As above, edit the hwclock.sh file and make sure Bios RTC wakeup is disabled.

Extra Information

Background to Advanced Configuration and Power Interface (ACPI)

The PC Industry have been moving from the older APM power managment specification to the newer ACPI specification. If your computer has a BIOS date more recent than 1999 and you are using a 2.6 kernel then ACPI support will most likely be enabled.

I have chosen to go with using ACPI to have my MythTV box wakeup while I am away , record a programme and then power itself back down. You will find the ACPI features for your machine exposed as a collection of files and directories in /proc/acpi. For instance /proc/acpi/alarm is a file that corresponds to the Real Time Clock wakeup function on your machine.

Note that beginning with Linux kernel 2.6.22, /proc/acpi/alarm has been deprecated and replaced with /sys/class/rtc/rtc0/wakealarm.

Activating ACPI support when compiling your own kernel

The setup of /proc/acpi/alarm is done in kernel file drivers/acpi/sleep/proc.c and controlled by the CONFIG_ACPI_SLEEP_PROC_FS flag.
You can test support for you architecture by going to you kernel source and do: